Process for removing sulfur particles from an aqueous catalyst solution and for removing hydrogen sulfide and recovering sulfur from a gas stream

ABSTRACT

The invention relates to a process for removing hydrogen sulfide and recovering sulfur from a gas stream, comprising the steps of: contacting said gas stream with an aqueous catalyst solution of a polyvalent metal redox catalyst in a contacting zone to absorb said hydrogen sulfide and form a reduced catalyst solution comprising reduced polyvalent metal redox catalyst and sulfur particles; oxidizing said reduced catalyst solution while removing sulfur particles to form said oxidized aqueous catalyst solution comprising polyvalent metal redox catalyst in an oxidized state with sulfur particles removed; and recovering sulfur by transferring at least one of said sulfur particles and foam to a separation zone; wherein a coagulating reagent is added to a feed of said separation zone prior to entering said separation zone to promote settlement of sulfur particles.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application from U.S. patentapplication Ser. No. 11/446,021 with the title ‘A process for removingsulfur particles from an aqueous catalyst solution and for removinghydrogen sulfide and recovering sulfur from a gas stream’ filed on Jun.1, 2006, which claims priority from European patent application no.05104812.2 with the title ‘A process for removing sulfur particles froman aqueous catalyst solution and for removing hydrogen sulfide andrecovering sulfur from a gas stream’ filed on Jun. 2, 2005, the wholecontent of which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process for removing hydrogen sulfide(H₂S) from gas streams, by using a redox process and removing theresulting sulfur particles from an aqueous catalyst solution used duringthe redox process.

BACKGROUND OF THE INVENTION

The presence of hydrogen sulfide in gas streams causes differentproblems in oil, gas and petrochemical industries and even wood anddrink industries. Removing H₂S from gas streams has, hence, changed to anecessary process. Furthermore, the regulations of environmentconservation organizations, on the permitted amounts of H₂S are gettingmore and more strict every day. This is mainly because of the poisonousand corrosive effects of H₂S.

Application of an aqueous solution of a polyvalent metal chelatecatalyst for the oxidative removing of H₂S, from gas streams and itsconversion to elemental sulfur has been well known. In these processes,H₂S-containing gas is contacted with, an aqueous solution of apolyvalent metal chelate in a contact zone. The contactor can be anysuitable device for gas—liquid contact, such as an absorber, a staticmixer, a Venturi scrubber, or even a combination of all. The aqueouscatalyst solution absorbs H₂S and converts it to elemental sulfurrapidly. As a result of this the higher oxidation state of the ion isreduced to its lower state. The lower oxidation state of the ion metalis then oxidized, as a result of contact with an oxygen containing gasto its higher oxidation state and is returned to the contact zone. Suchreactions are called liquid Redox reactions. The separation of the solidparticles can take place either before or after regenerating of thepolyvalent metal chelate.

Due to settling or sedimentation of sulfur particles, however, thecontactor can become plugged. If the settling happens to a great extentand the tower is plugged, this will perturb the contactor, in particularan absorber, and it will ultimately flood. In the case of theapplication of an absorber, filled with a packing, the choice of thepacking material is also important. Usually a suitable packing has to beselected, which results in the least setting of solid sulfur particles.Often, such a packing does not exhibit optimum reaction rates.

U.S. Pat. No. 4,784,754 discloses a process for removing sulfurparticles from an aqueous polyvalent metal ion or polyvalent metalchelate solution by a method of sinking the sulfur particles (gravitysedimentation) in a zone. In order to reduce foam and/or froth floatingon a surface of the zone, sulfur particles suspended as a froth or foamare agitated and removed in a plurality of streams including at leastone stream at a short distance from the top of the solution in the zoneand at least one stream from the bottom of the solution in the zone.Subsequently, the streams are recombined for further processing.

U.S. Pat. No. 4,816,238 discloses another process for the removal ofhydrogen sulfide from a sour gaseous stream, wherein an aqueous alkalinesolution is contacted with a polyvalent metal chelate in a highervalence state in order to oxidize the hydrogen sulfide or sulfidepresent to sulfur. Particular measures for preventing clogging of sulfurparticles in the oxidizer are not disclosed.

EP 0 582 337 A1 discloses another process for removing hydrogen sulfidefrom a gas mixture. U.S. Pat. No. 5,122,351 discloses another processfor removing hydrogen sulfide from a process gas, wherein a closed loopevaporator/condenser process is interposed in the sulfurwashing/filtering/recovery process in order to recover and re-use acatalytic polyvalent metal redox solution. Wash water used to purify thesulfur and any polyvalent metal redox solution recovered from a sulfurmelter are fed to an evaporator to concentrate the redox solution to aconcentration capable of effective absorption of hydrogen sulfide.Furthermore, the water evaporated in the evaporator is condensed as purewater for use in washing and/or filtering the recovered sulfur.Particular measures for preventing clogging of sulfur particles in theoxidizer are not disclosed.

EP 0 186 235 A1 discloses a process for removal of acid gases from asour gaseous stream. In the process a sour gaseous stream comprising H₂Sis contacted in a column with an aqueous reactant solution comprising aneffective amount of Fe(III) chelate of an organic acid to obtain a sweetgaseous stream and a mixture including solid sulfur and Fe(II) chelateof the acid. Degradation of the iron chelate in the reactant solutionemployed in the cyclic process is inhibited by maintaining a relativelyhigh Fe(II) chelate concentration by carrying out the regeneration stepin the column as a plug flow contracting procedure. The problem offoaming and flooding of the oxidizer is not discussed specifically. Theflow state in the oxidizer zone is not addressed specifically.Separation of the sulfur particles takes place in a separate vessel.

U.S. Pat. No. 6,596,253 B1 discloses a process for desulfurization of agaseous feed containing hydrogen sulfide. The sulfur particles and thereduced catalyst solution are separated in a preliminary step and thestream of reduced catalyst solution is sent to a downstream oxidizer.

General principles for use of ferric chelates for the oxidization ofhydrogen sulfide are disclosed in Iliuta I., et al., ‘Concept ofbifunctional Redox iron-chelate process for H2S removal in pulp andpaper atmospheric emissions’, Chemical Engineering Science, Oxford, GB,volume 58, no. 34-24, December 2003, pages 5305-5314.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a more efficient,reliable and economical process for removing hydrogen sulfide andrecovering sulfur from a gas stream. According to another aspect of theinvention excessive foaming and flooding of the oxidizer zone is to beavoided in such a process. It is another object of the present inventionto provide a more efficient, reliable and economical process forremoving sulfur particles from an aqueous catalyst solution used toremove hydrogen sulfide from a gas stream.

According to a first aspect of the present invention there is provided aprocess for removing hydrogen sulfide and recovering sulfur from a gasstream, comprising the steps of: contacting said gas stream with anaqueous catalyst solution of a polyvalent metal redox catalyst in acontacting zone to absorb said hydrogen sulfide and form a reducedcatalyst solution comprising reduced polyvalent metal redox catalyst andsulfur particles; oxidizing said reduced catalyst solution whileremoving sulfur particles by a method as outlined above to form aregenerated or oxidized aqueous catalyst solution comprising polyvalentmetal redox catalyst in an oxidized state with sulfur particles removed;and recovering sulfur by transferring said sulfur particles and/or foamto a separation zone; wherein a coagulating reagent is added to a feedof said separation zone prior to entering said separation zone topromote settlement of sulfur particles.

According to another embodiment said coagulating reagent is acryl amideand is added to said feed a predetermined time interval before enteringsaid separation zone, preferably by 1-3 seconds.

According to another embodiment heavy hydrocarbons and/or water areremoved from said gas stream in a gas/liquid separation means and saidtreated gas stream is cooled to a predetermined temperature range beforesaid step of contacting said gas stream with said aqueous catalystsolution is performed.

According to another embodiment said gas stream is contacted with saidaqueous catalyst solution in a Venturi scrubber.

According to another embodiment said gas stream is contacted with saidaqueous catalyst solution in an absorber containing a packing material,where a mass transfer takes place between a liquid film of said aqueouscatalyst solution formed on the packing material and said gas streambubbling through the packing material.

According to another embodiment the packing material is at leastpartially a random packing consisting of metal packing members,preferably of stainless steel elements. These metal packing members canbe shaped like hollow ring-shaped members having concavely curvedwing-shaped members extending inwards in radial direction. These metalpacking members may be packed at predetermined portions of the absorber,preferably as a random packing, whereas the remaining portions of theabsorber may be free of such metal packing elements.

According to another embodiment a pressure difference between an inletand an outlet of said contacting zone is regulated to a predeterminedpressure range to control the inlet flow of gas to said contacting zone.

According to another embodiment said flow of a suspension is heated orcooled to a predetermined temperature range before entering saidoxidizer zone.

According to another embodiment said temperature range is below adegradation temperature of said polyvalent metal redox catalyst,preferably in the range between 30° C. and 50° C., in particular in arange in which the catalytic regeneration is also high.

According to another aspect of the present invention that can also becombined with any other embodiment disclosed herein there is provided aprocess for removing sulfur particles from an aqueous catalyst solutionused to remove hydrogen sulfide from a gas stream, comprising the stepsof: directing a flow of a suspension comprising reduced catalystsolution and sulfur particles to an oxidizer zone, where the catalystsolution is regenerated by contacting said suspension with a gascontaining oxygen; and removing sulfur from said suspension at least bygravity sedimentation at a bottom of said oxidizer zone; wherein a flowdeflecting means is disposed at least at an outlet for the oxidizedcatalyst solution leaving the oxidizer zone for reducing any turbulentstate caused at least by a stream of oxidized catalyst solution leavingsaid oxidizer zone such as to reduce foaming and plugging of the wholesystem. Thus, excessive foaming is prevented according to the presentinvention. Furthermore, gravity sedimentation of sulfur particles ispromoted.

According to another embodiment the flow deflecting means is a baffledisposed under an acute angle with a circumferential surface or wall ofthe vessel surface of said oxidizer zone such that said streams ofoxidized catalyst solution leaving said oxidizer zone are deflected toanother direction before leaving said oxidizer zone. Of course, the flowof oxidized catalyst solution leaving the oxidizer zone can be deflectedsuch that the streams within the liquid phase of the oxidizer zone aresupported, in particular such that the streams within the liquid phaseof the oxidizer zone remain in a laminar state. Further, according tothis embodiment the baffle means effectively shields the outlet for thereduced catalyst solution as to prevent the direct transfer of solidsulfur particles during sedimentation within the oxidizer zone into thestream of oxidized catalyst solution leaving the oxidizer zone.

According to further embodiments, the afore-mentioned flow deflectingmeans may also be disposed alternatively or additionally at any inlet oroutlet where streams enter or leave the oxidizer zone, in particular alower section thereof used for sedimentation of sulfur particles bygravity, such as to reduce foaming and plugging of the whole system.

According to another embodiment said oxidizer zone is a bubble column,the gas containing oxygen is bubbled into said column by means of atleast one sparger so that lighter sulfur particles go up to an uppersurface of a liquid phase within said column, and said flow ofsuspension enters said column at a position vertically disposed at ashort distance below said upper surface. Thus, the convection-like rollof streams in the liquid phase within the oxidizer zone is furthersupported.

According to another aspect of the present invention that can also becombined with any other embodiment disclosed herein there is provided aprocess for removing sulfur particles from an aqueous catalyst solutionused to remove hydrogen sulfide from a gas stream, comprising the stepsof: directing a flow of a suspension comprising reduced catalystsolution and sulfur particles to an oxidizer zone, where the catalystsolution is regenerated by contacting said suspension with a gascontaining oxygen; and removing sulfur from said suspension at least bygravity sedimentation at a bottom of said oxidizer zone; wherein thebottom of the oxidizer zone comprises a downwardly slanted surface and agas is additionally injected at said bottom of said oxidizer zonesubstantially in parallel or tangentially to said slanted surface foravoiding sedimentation and agglomeration of sulfur particles on saidbottom. According to this aspect of the invention, any settled sulfurmay be ‘blown’ or pushed away towards the center part of the bottom partwhere an outlet for transferring the slurry to a subsequent processstage is provided.

According to another embodiment lighter sulfur particles and/or foamfloating on a surface of said liquid phase are collected by a sweeperrotating at low speed and removed from said oxidizer zone for furtherprocessing.

According to another aspect of the present invention that can also becombined with any other embodiment disclosed herein there is provided aprocess for removing sulfur particles from an aqueous catalyst solutionused to remove hydrogen sulfide from a gas stream, comprising the stepsof: directing a flow of a suspension comprising reduced catalystsolution and sulfur particles to an oxidizer zone, where the catalystsolution is regenerated by contacting said suspension with a gascontaining oxygen; and removing sulfur from said suspension at least bygravity sedimentation at a bottom of said oxidizer zone; wherein lightersulfur particles and/or foam are collected simultaneously to removal ofheavier sulfur particles at said bottom of said oxidizer zone and theremoved sulfur particles and/or foam are transferred for furtherprocessing continuously or in a batch wise mode. Thus, by sweeping ofthe lighter sulfur particles as foam in the top part of the oxidizerzone and simultaneous settlement and carrying away of the heavier sulfurparticles in the bottom part of the oxidizer zone plugging and foamingis substantially reduced according to the present invention.

As will become apparent to a person skilled in the art, according to aparticularly preferred embodiment of the present invention, any of thefollowing measures, as explained in more detail above, can contribute ina particular combinatorial manner to substantially reduce plugging andfoaming in a process for removing sulfur particles from an aqueouscatalyst solution used to remove hydrogen sulfide from a gas stream.Such measures are in particular:

-   -   using simultaneous settlement and floatation separations; and/or    -   adding a coagulant in a proper time and place; and/or    -   use of a sweeper; and/or    -   use of a special packing in the absorber zone; and/or    -   use of flow deflecting means, in particular baffles, in the        oxidizer zone.

BRIEF DESCRIPTION OF DRAWINGS

Hereinafter, exemplary embodiments according to the present inventionwill be described with reference to the accompanying drawings, fromwhich further features, advantages and objects will become apparent andwherein:

FIG. 1 shows a schematic view for illustrating a process according tothe present invention for removing hydrogen sulfide and recoveringsulfur from a gas stream;

FIG. 2 is an enlarged partial sectional view of the oxidizer zoneaccording to FIG. 1 showing an example for a baffle provided near theoutlet of the reduced catalyst solution leaving the oxidizer zone;

FIG. 3 is an enlarged partial sectional view of the oxidizer zoneaccording to another embodiment according to the present inventionshowing an example for a sweeper disposed at an upper end of theoxidizer zone;

FIG. 4 is an enlarged sectional view of the bottom portion of theoxidizer zone according to another embodiment according to the presentinvention showing the flow of additional gas essentially tangential tothe slanted bottom surface of the oxidizer zone; and

FIG. 5 shows an example for a metal packing element used in the absorberaccording to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention relates to the reduction of foaming and pluggingin processes where H₂S is separated from H₂S-containing gas streams, bybeing contacted with an aqueous catalyst solution comprising a chelateof a polyvalent metal ion, which is preferably an iron chelate.

As shown in FIG. 1, the overall process comprises three major sections,namely the contact zone 6, the oxidizer zone 20 and the separator zone40. As shown in FIG. 1, a sour or acid gas 1 is fed via a heat exchangerE1 and line 2 to a gas-liquid-separator 3, which separates any liquidheavy hydrocarbons and water to line 4, as outlined further below. Theseparated gaseous hydrocarbons are fed via line 5, Venturi scrubber 10and line 8 to the contact zone 6. More specifically, the acid or sourgas stream 8 enters the contact zone 6 at a bottom part thereof.

The contact zone 6 can be an absorber, a static mixer, a Venturiscrubber or a combination thereof. Preferably, according to the presentinvention the contact zone 6 is a Venturi scrubber. In the contact zone6, the H₂S-containing gas stream, which enters the contact zone 6 viathe inlet 8, is contacted with the aqueous solution of the catalyst,containing the polyvalent metal ion chelate, at temperatures lower thanthe melting point of sulfur. According to this embodiment, thepolyvalent metal ion chelate is preferably a Fe³⁺ chelate, although thepresent invention is not limited thereto. As a result of the contact,the higher oxidation state of the metal ion (Fe³⁺ in the case of an ironchelate) is reduced to its lower oxidation state (reduced state; Fe²⁺ inthe case of an iron chelate), and the ionic sulfur is oxidized to itselemental form, according to the following reactions (1-3):H₂S_((g))+H₂O_((liq))→H₂S_((aq))  (1)H₂S_((aq)){right arrow over (←)}HS⁻ _((aq))+H⁺ _((aq))  (2)HS⁻ _((aq))+Fe³⁺ _((aq))→S⁰ _((sol))+2Fe²⁺ _((aq))+H⁺ _((aq))  (3)In this process, solid sulfur particles are formed. As a result of thisfast process about 90% of the sulfur content of the gas stream 8 isremoved. The mixture of the solid sulfur particles and of the differentoxidation states of the metal ion chelate (a mixture of Fe²⁺ and Fe³⁺ inthe case of the application of the iron chelate), forming a slurry,leaves the contact zone 6 via line 12 at the bottom of the contact zone6 and is pumped, by means of pump P1, to the inlet 13 of the oxidizerzone 20, which will be described in more detail below. The substantiallyH₂S-free gas leaves the contact zone 6 via the outlet line 11.

More specifically, the substantially H₂S-free gas starts an upwardmovement in the contact zone 6, where some aqueous catalyst is injectedto the contact zone 6 from its top part via inlet 7. Thus, the aqueouscatalyst solution flows down by gravity while the gas risescounter-currently through the contact zone 6. In the contact zone 6 amass transfer takes place across the gas-liquid interface thus formed.Thereby, the probability of the formation of the black FeS impurity isminimized, so that it starts a downward movement, opposite to that ofthe up-going gas, eliminating the remaining 10% of the sulfur.

Flooding of the system, in particular of the contact zone 6, can also becontrolled and prevented by using a cascade control system. The controlsystem holds the Δp of the system (the pressure difference between theoutlet 11 and the inlet 8 of the contact zone 6) at a certainpredetermined value by varying the flow of hydrocarbon gas stream 5.

Extensive experiments of the inventors surprisingly have revealed thatthe presence of high amounts of heavy hydrocarbons in the absorber canalso lead to foaming and the consequent flooding in the various stagesof the process, in particular in the oxidizer zone 20, because heavyhydrocarbons change the surface tension. For preventing high amounts ofheavy hydrocarbons, more specifically for preventing excessive foamingand the resulting flooding in the various stages of the process, theH₂S-containing gas stream 1 is optionally passed through a cooler E1 tokeep the temperature of the H₂S-containing gas stream 1 within asuitable range, preferably in the temperature range between 30° C. and40° C., and is then passed through a gas-liquid-separator 3 forseparating liquid particles from the gas stream 2. The gas-liquidseparator 3 can rely on any of the known concepts of mechanicalgas-liquid separation, namely gravity (knock-out) separation,centrifugal (cyclone) separation, separation by impingement andseparation by filters. More preferably, according to the presentinvention the gas-liquid separator is a gravity or knock-out drum havinginlet and outlet connections located on the upper portion of a vessel.The force used to separate the liquid heavy hydrocarbons from the gasstream 2 is gravity. The gas velocity should be relatively low in thisembodiment in order for separation to occur. After separation, the heavyhydrocarbon content which is in a liquid state exits via outlet 4 at thebottom of the drum 3 to thereby prevent the subsequent flooding of thecontact zone 6.

The slurry formed at the bottom of the contact zone 6, which is thespent or reduced catalyst solution together with solid sulfur particles,from both the contact zone 6 and the top part of thereof, is pumped, bypump P1, to heat exchanger E2, before it enters the oxidizer zone 20.The heat exchanger E2 adjusts the temperature of the slurry to atemperature well below a degradation temperature of the catalystsolution, above which the catalyst degradation is relatively high. Morespecifically, the heat exchanger E2 keeps the temperature of the slurrya predetermined amount below the degradation temperature. In theexemplary embodiment, where polyvalent iron chelate is used as acatalyst, the temperature is adjusted in the range between 30° C. and50° C., i.e. at a temperature lower than a temperature where theregeneration process is substantially slowed down and not higher than50° C., above which temperature the catalyst degradation is relativelyhigh.

According to another embodiment the contact zone 6 is an absorber. Asschematically indicated by reference numeral 15, the absorber maycontain a packing of a proper physical structure which will reduce thesettlement of the solid sulfur particles that are formed during theprocess on the packing, which may cause plugging of the absorber,turbulancy of the streams and finally the flooding of the absorber.According to the present invention, such a packing consists of aplurality of packing elements that will be described in more detail withreference to FIG. 5, which shows an exemplary embodiment of a packingelement used according to the present invention.

As shown in FIG. 5, the packing element 50 consists of two ring members51 that are interconnected by webs 52 that form concavely curvedwing-shaped surfaces that extend radially inwards. As shown in FIG. 5,the free ends of the webs 52 are adjacent to each other but do notcontact each other, thereby leaving a free space. Thus, the packingelements 50 have a very high surface area to be wetted by the catalystsolution. The packing elements let the up-moving gas stream face a muchlarger surface area wetted by the catalyst solution. The remainingsulfur content of the gas stream reacts with the down-going catalystsolution, on the wetted packing, and approximately all of its sulfurcontent is removed in this way. The approximately completely H₂S-freegas leaves from the top part of the absorber 6 via outlet 11, while thedown moving reduced catalyst solution joins the other reduced solution,which is from the Venturi scrubber, at the bottom of the absorber 6.Extensive experiments of the inventors proved that application ofpacking material as schematically shown in FIG. 5, preferably ofstainless steel, leads to the least settlement of the solid particles,thereby reducing the probability of flooding of the system.

The packing is also put in the system in a way that it results in aminimum pressure drop in the system. The lower pressure drop moves theoperating point farther away from the critical pressure drop at whichflooding occurs (typically 10-20 mbar/m), which means additionalvapor/liquid handling capability. This is done by putting the packing ina structured way in the three ends of the absorber 6.

More specifically, according to another preferred embodiment accordingto the present invention a part of the absorber 6 is filled with astructured packing whereas the remaining part of the absorber 6 isfilled with a random packing as schematically shown in FIG. 5. In suchan embodiment the random packing may have an aspect ratio of only 1:3,i.e. its height is typically only ⅓ of its diameter, which has aprofound effect on the process performance of the packing. In operation,the flat ring-shaped packing members tend to orient themselvespreferentially in a ‘near-horizontal’ position, i.e. with theircylindrical axis predominantly in the direction of the gas and liquidflow, which results in a lower pressure drop due to an easier gaspassage, and in a higher capacity. According to another preferredembodiment, the ring-shaped packing members are arranged in cylindricalpassages with an outer diameter of about 5 cm. Those parts with astructured packing may be provided adjacent in horizontal or verticaldirection of the absorber 6.

As will become apparent to the person skilled in the art, the presentinvention is not limited to the use of random packing of the typeschematically shown in FIG. 5 as a packing for the absorber 6. Ingeneral, other packings may also be used, e.g. conjugate ring packings,VSP ring packings, ball ring packings, saddles, Teller packings, rosettepackings, helix packings, polyhedral hollow ball packings. Otherstructured packings that are also contemplated for use according to thepresent invention include oblique, gauze, perforated and corrugatedplate packings.

In the following, the oxidizer zone according to the present inventionwill be described in more detail. As shown in FIG. 1, the oxidizer isformed as a cylindrical vessel 20 having a conical bottom 21. The slurryexiting the bottom of the contact zone 6, i.e. the spent or reducedcatalyst solution together with solid sulfur particles, enters theoxidizer zone 20 via inlet 13 and via an outer circumferential surfaceof the cylindrical vessel 20, below the top surface of the liquid phasewithin the vessel 20. The temperature of the slurry is adjusted in theoptimum range by the heat exchanger E2, as outlined above.

Air or another gas containing oxygen is blown into the cylindrical partof vessel 20 via line 22, suitably at a distance to the transitionregion between the bottom part 21 and the cylindrical part of the vessel20. In the vessel 20 as result of the reaction of the reduced catalystwith the oxygen containing gas, e.g. air, the lower oxidation state ofthe metal ion in the chelate (Fe²⁺ in the case of an iron chelate) isoxidized to its higher oxidation state (Fe³⁺ in the case of an ironchelate). The reactions can be summarized by (4) and (5) as follows:O_(2(g))+2H₂O₍₁₎→O_(2(aq))  (4)O_(2(aq))+4Fe_((aq)) ²⁺+H₂O₍₁₎→4Fe_((aq)) ³⁺+4OH_((aq)) ⁻  (5)

More specifically, air bubbles are blown into the oxidizer zone 20 bymeans of a sparger 23, which is connected with line 22. The oxidizerzone 20 is preferably a bubbling column partially filed with the liquidphase, i.e. the reduced catalyst solution and the slurry, into which airor another air containing gas is bubbled.

The sparger 23 induces an upward movement of air bubbles in the liquidphase. Furthermore, also some very small (lighter) sulfur particles 25go up (rise) in the liquid phase within the vessel 20. As an additionalcomponent also heavy hydrocarbons, no matter how it has found its way tothe oxidizer, contribute to this upward stream within the vessel 20.Thereby, a foam is generated on the top surface of the liquid phasewithin the oxidizer 20, which is to be avoided according to the presentinvention.

To reduce the effects of foaming, a low speed sweeper 29 driven by anelectromotor 30 having a speed of 5-10 rounds per minute (rpm) isprovided at the top end of the vessel 20. When rotating, the sweeper 29collects any foam generated on the top surface of the liquid phase andleads it to a channel 37 (FIG. 3) to transfer the foam, via line 37′, toa coalescer (V3) wherein the first steps of the separation process areperformed and which will be described in more detail below.

As shown in FIG. 3, the bottom of the sweeper 29 is flush with thesurface of the liquid phase within the oxidizer 20. At a peripheralportion of the oxidizer 20, a channel 37 is formed by a vertical wall,whose top end is substantially flush with the surface of the liquidphase within the oxidizer 20, and a horizontal wall. Thus, any foamgenerated on the surface of the liquid phase within the oxidizer 20 ispushed by the rotating sweeper 29 over the top edge of the vertical wallinto channel 37, from where the foam is guided, via line 37′, to thecoalescer.

As shown in FIG. 1, the oxidizer 20 is composed of two virtuallyseparated sections, namely the upper first section above the sparger 23and the lower second section below the sparger 23. In the upper firstsection the reaction between the catalyst and the air containing gasstreams happens. In this first section, according to the chemicalprinciples involved, turbulancy of the fluid streams is useful becauseit increases the reaction rate and efficiency. As shown in FIG. 1, thevolume and dimensions of this first section are relatively largecompared to the amount of the catalyst flow. On the other hand, thesecond lower section acts as a ‘sulfur settling’ section or ‘catalystclarifier’ section, where the sulfur particles are separated from thecatalyst streams. In this section, the turbulancy is reduced so that theheavier sulfur particles can settle and the lighter ones can go up tothe surface of the liquid held in the vessel 20. In order to reduceturbulent effects in the lower section of the oxidizer in the presentinvention, the application of flow deflecting means, particularly theapplication of special baffles, is proposed. As a result of the use offlow deflecting means, in particular baffles, and also taking othermeasures like simultaneous settlement and floating separations, adding acoagulant in a proper time and place, use of a sweeper, and use of aspecial packing in the absorber zone, as explained in more detail inthis invention, the plugging and foaming effects in the whole system arereduced.

On the other hand, the larger sulfur particles 26 or those enlarged as aresult of attaching to one another (agglomeration) have a tendency tosettle in the oxidizer zone 20 (sedimentation by gravity). Extensiveexperiments of the inventors revealed that one of the measures to reduceplugging and foaming effects in the whole system is to reduce anyturbulent state in the lower second section of the oxidizer zone 20.

In order to reduce any significant disturbing effect of the stream ofoxidized catalyst solution leaving the oxidizer zone 20 on the streamswithin the liquid phase in the oxidizer zone 20, according to thepresent invention a flow deflecting plate or similar means is disposedin the region near the outlet 35, in a manner similar to that to bedescribed in more detail with reference to FIG. 2 below.

Referring to FIG. 2, the flow deflecting plate 34 consists of asubstantially slanted portion 34, which extends under an acute anglesubstantially in a vertically upward direction towards the top of line35. Thus, according to this embodiment the flow of oxidized catalystsolution leaving the oxidizer zone 20 is smoothly deflected in asubstantially horizontal direction so that the streams of liquid in theoxidizer zone 20 are substantially not effected and in particular noturbulent state is caused. Furthermore, the flow deflecting plate 34also shields the orifice of line 35 from solid sulfur particles settlingin the oxidizer zone 20. Thus, the flow deflecting plate 34 effectivelyprevents the direct flow of settling sulfur particles into the stream ofoxidized catalyst solution leaving the oxidizer zone 20 via outlet line35. As shown in FIG. 2, the flow deflecting plate 34 substantiallycovers the entire cross section of line 35. As will become apparent to aperson skilled in the art, the flow deflecting plate 34 may, of course,also be curved in a convex manner. As no turbulent state is induced inthe liquid phase in the oxidizer zone 20, settlement process(sedimentation by gravity) of the larger sulfur particles 26 is mademore efficient.

As will become apparent to a person skilled in the art, a plurality ofsuch flow deflecting plates can also be disposed at equiangulardistances around the circumference of the cylindrical vessel 20.

According to a further embodiment (not shown), a similar flow deflectingplate may also be disposed within the path or stream of the slurryflowing out of the line 13 and into the oxidizer zone 20. Thus,according to this embodiment the slurry flow cannot directly enter theoxidizer zone 20 but is smoothly deflected into another direction sothat the slurry flow entering the oxidizer zone 20 will not disturb thestreams within the liquid phase in the oxidizer zone 20 significantly.In particular, the slurry flow will not cause further turbulent effectsin the liquid phase.

In the following, the agglomeration of sulfur particles at the bottom ofthe oxidizer zone will be discussed in more detail with reference toFIG. 1 and FIG. 4. Extensive experiments of the inventors revealed also,that the solid sulfur particles 26, which tend to settle under theconditions as outlined above, will attach to the conical bottom 21 ofthe oxidizer zone 20. Agglomeration of the heavier sulfur particles 26at the conical bottom 21 does not only make their transfer to thesubsequent processing stage (the coalescer 40) very difficult, if notimpossible, but also results in clogging and even flooding of theoxidizer zone 20, which is to be avoided.

To avoid this, according to another aspect of the present invention airstreams 27 having a proper strength are injected in parallel to thedownwardly slanted segments 31 of the conical part 21 of the oxidizer20, as shown in more detail in FIG. 4. Thus, the heavier sulfurparticles 26 are separated from the conical bottom 21 of the oxidizerzone 20 and driven to the outlet line 36 at the center of the conicalbottom 21. These settled particles and the floating ones aretransferred, via line 36 and pump P2, to the next separation stage 40either simultaneously or at different timings. They can also betransferred continuously or in a batch-wise mode.

As shown in FIG. 4, a flow deflecting plate 32 is disposed at theorifice of line 27 supplying the stream of air to the bottom part of theoxidizer zone. More specifically, the flow deflecting plate comprises asubstantially horizontal portion extending in radial inward direction aswell as a slanted portion 32, which extends downwardly in a slantedmanner and substantially in parallel with the slanted bottom surface 31.Thus, the flow of incoming air, or of an inert gas, is deflected into adirection substantially in parallel with or tangential to the slantedbottom surface 31, as indicated by the arrow. As will become apparent toa person skilled in the art, the gap between the inner surface of theflow deflecting plate 32 and the slanted bottom surface 31 may berelatively small as long as a stream of air of sufficient strength isobtainable.

According to another embodiment (not show), the flow deflecting plate 32may also be directly attached to the slanted bottom surface as toprevent the direct entrance of the stream of air into the conical bottompart of the oxidizer zone. For causing the stream of incoming air into adirection substantially in parallel with the slanted bottom surface, acurved recessed portion may be provided at the bottom end of the inlet27, so that the stream of air first impinges onto the plate and reflectsback to the recession, which finally deflects the air stream in adirection substantially in parallel with or tangential to the slantedbottom surface 31.

According to another embodiment (not shown), the flow deflecting platemay also be disposed spaced apart from and substantially in parallelwith the slanted bottom surface. The flow deflecting plate may preventthe direct entrance of air into the conical bottom part of the oxidizerzone and may divide the incoming stream of air into a first portion,which is deflected into an upward direction and substantially inparallel with or tangential to the slanted bottom surface, and anotherportion, which is deflected into a downward direction and substantiallyin parallel with or tangential to the slanted bottom surface.

As will become apparent to a person skilled in the art, a plurality ofsuch flow deflecting plates can be disposed at equiangular distancesaround the circumference of the slanted bottom surface 31.

As shown in FIG. 1, the next separation stage is a coalescer 40 wherecoagulating reagent is added that helps the settlement process to beperformed more efficiently. According to the present invention thecoagulating reagent is preferably acryl amide, which enhances separationsignificantly. It is noteworthy that the larger sulfur particlestransferred to the coalescer 40 play the role of nuclei in thesubsequent nucleation process that will happen in the coalescer, whichresults in a significantly higher efficiency. Also the time and place ofthe addition of the coagulating reagent has a great impact on theseparating and recovery efficiency. The inventors observed that the bestsettlement is achieved when the coagulating reagent is added 1-3 secondsbefore the entrance of the slurry to coalescer 40, which can beimplemented e.g. by means of a valve through which the coagulatingreagent is injected into line 36 before the slurry enters the coalescer40. Settled (sedimented) sulfur particles are transferred via line 41 tothe separation zone 42, which can be any of the various filter elementsand preferably a rotary vacuum filter. After the filtration process theamount of the solid particles present in the filtrate 43, which is goingto be recycled, is considerably reduced to thereby avoid plugging to alarge extent. Recovered catalyst solution is transferred via line 44back to the oxidizer zone 20.

Also, by the separation of a fraction of about 15-20% of the circulatingstream, production costs are reduced, which could not be possible if theseparation would be performed e. g. before the regeneration zone.

As will become apparent to a person skilled in the art, according to aparticularly preferred embodiment of the present invention, any of thefollowing measures, as explained in more detail above, can contribute ina particular combinatorial manner to substantially reduce plugging andfoaming in a process for removing sulfur particles from an aqueouscatalyst solution used to remove hydrogen sulfide from a gas stream, asoutlined above:

-   -   using simultaneous settlement and floatation separations; and/or    -   adding a coagulant in a proper time and place; and/or    -   use of a sweeper; and/or    -   use of a special packing in the absorber zone; and/or    -   use of flow deflecting means, in particular baffles, in the        oxidizer zone.

In accordance with the present invention a solution of a polyvalentmetal in chelate form is contacted with the hydrogen sulfide-containinggas. The chelate solution, per se, may be selected from among thechelate solutions taught by the art to be useful in sulfur oxidationprocesses. Further, the metals which may be employed are thosepolyvalent metals which will oxidize hydrogen sulfide to sulfur and inturn be reoxidized by oxygen or similar gas. These metals are used withproper adjustment in concentration. Any polyvalent metal can be used,but iron, copper and manganese are preferred, particularly iron. Thepolyvalent metal should be capable of oxidizing hydrogen sulfide, whilebeing reduced itself from a higher to a lower valence state, and shouldthen be oxidizable by oxygen from the lower valence state to the highervalence state in a typical redox reaction. Other polyvalent metals whichcan be used include lead, mercury, palladium, platinum, tungsten,nickel, chromium, cobalt, vanadium, titanium, tantalum, zirconium,molybdenum and tin. The metals are normally supplied as a salt, oxide,hydroxide etc.

The chelating agents or liquids which may be used together withpolyvalent metallic cations are those which form a complex ion havingstability in solution. These compounds may be of any substance whichwill effectively complex the metal ion by forming cyclic structures.These materials include aminopolycarboxylic acid chelating agents of thealkylenediamine and phenylene-diamine types, such as ethylendediaminetetracetic acid, nitrilotriacetic acid, or the like. They may alsocontain ammonia or an aliphatic, alicyclic or heterocyclic primary orsecondary amine.

As will become apparent to the person skilled in the art, the term‘zone’ as employed in the specification and appended claims includes,where suitable, the use of segmented equipment operated in series, orthe division of one unit into multiple units because of sizeconstraints, etc. E.g. a contacting column or absorption column mightcomprise two separate columns in which the solution from the lowerportion of the first column would be introduced into the upper portionof the second column, the gaseous material from the upper portion of thefirst column being fed into the lower portion of the second column.Parallel operation of units is, of course, well within the scope of thepresent invention.

Thus, according to the present invention the following main advantagesare achieved:

-   -   foam-producing heavy hydrocarbons can be eliminated;    -   the foaming and plugging is effectively controlled by using        proper packings in the absorber;    -   proper controls are used to avoid flooding of the oxidizer;    -   the temperature is adjusted such that not only the reaction rate        in the oxidizer is high, but also the catalyst degradation is        kept low.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention.

1. A process for removing hydrogen sulfide and recovering sulfur from agas stream, comprising the steps of: contacting said gas stream with anaqueous catalyst solution of a polyvalent metal redox catalyst in acontacting zone to absorb said hydrogen sulfide and form a reducedcatalyst solution comprising reduced polyvalent metal redox catalyst andsulfur particles; oxidizing said reduced catalyst solution whileremoving sulfur particles to form said oxidized aqueous catalystsolution comprising polyvalent metal redox catalyst in an oxidized statewith sulfur particles removed; and recovering sulfur by transferring atleast one of said sulfur particles and foam to a separation zone;wherein a coagulating reagent is added to a feed of said separation zoneprior to entering said separation zone to promote settlement of sulfurparticles.
 2. The process as claimed in claim 1, wherein said step ofoxidizing said reduced catalyst solution while removing sulfur particlescomprises the steps of: directing a flow of a suspension comprisingreduced catalyst solution and sulfur particles to an oxidizer zone,where the catalyst solution is regenerated by contacting said suspensionwith a gas containing oxygen; and removing sulfur from said suspensionat least by gravity sedimentation at a bottom of said oxidizer zone. 3.The process as claimed in claim 1, wherein said coagulating reagent isacryl amide.
 4. The process as claimed in claim 3, wherein said acrylamide is added to said feed a predetermined time interval beforeentering said separation zone, preferably by 1-3 seconds.
 5. The processas claimed in claim 1, wherein at least one of heavy hydrocarbons andwater are removed from said gas stream in a gas/liquid separation meansand said treated gas stream is cooled to a predetermined temperaturerange before said step of contacting said gas stream with said aqueouscatalyst solution is performed.
 6. The process as claimed in claim 5,wherein said gas stream is contacted with said aqueous catalyst solutionin a Venturi scrubber.
 7. The process as claimed in claim 5, whereinsaid gas stream is contacted with said aqueous catalyst solution in anabsorber containing a packing material, where a mass transfer takesplace between a liquid film of said aqueous catalyst solution formed onthe packing material and said gas stream bubbling through the packingmaterial.
 8. The process as claimed in claim 7, wherein said packingmaterial is at least partially a random packing consisting of aplurality of metal packing members, preferably of stainless steelelements.
 9. The process as claimed in claim 1, wherein a pressuredifference between an inlet and an outlet of said contacting zone isregulated to a predetermined pressure range to control the inlet flow ofgas to said contacting zone.
 10. The process as claimed in claim 1,wherein said flow of a suspension is heated or cooled to a predeterminedtemperature range before entering said oxidizer zone.
 11. The process asclaimed in claim 10, wherein said temperature range is below adegradation temperature of said polyvalent metal redox catalyst,preferably in the optimum range between 30° C. and 50° C., in which thecatalytic regeneration is also high.
 12. The process as claimed in claim2, further comprising the step of disposing a flow deflecting means atleast at an outlet for the oxidized catalyst solution leaving saidoxidizer zone for reducing any turbulent state caused at least by astream of oxidized catalyst solution leaving said oxidizer zone such asto reduce foaming and plugging of the whole system.
 13. The process asclaimed in claim 12, wherein said flow deflecting means is a baffleforming an acute angle with a circumferential surface of said oxidizerzone such that said stream of oxidized catalyst solution is deflected toanother flow direction.
 14. The process as claimed in claim 2, whereinsaid oxidizer zone is a bubble column, said gas containing oxygen isbubbled into said column by means of at least one sparger so thatlighter sulfur particles go up to an upper surface of a liquid phasewithin said column, and said flow of suspension enters said column at aposition vertically disposed below said upper surface.
 15. The processas claimed in claim 1, further comprising the step of directing a flowof a suspension comprising reduced catalyst solution and sulfurparticles to an oxidizer zone, where the catalyst solution isregenerated by contacting said suspension with a gas containing oxygen;wherein said bottom comprises a slanted surface and a gas isadditionally injected at said bottom of said oxidizer zone substantiallyin parallel to said slanted surface for avoiding sedimentation andagglomeration of sulfur particles on said bottom.
 16. The process asclaimed in claim 12, wherein at least one of lighter sulfur particlesand foam floating on a surface of said liquid phase are collected by arotating sweeper and removed from said oxidizer zone for furtherprocessing.